In certain mechanisms, an operating shaft is driven rotatably in one direction, or either direction, from a reference position through a limited angle of displacement and is required to return to the reference position after release of the driving torque. For this purpose, it is a common practice to provide a return spring which is coupled with the shaft in such manner that it is stressed by rotation of the shaft away from the reference position. When the driving torque is removed from the shaft, the stored energy of the return spring drives the shaft toward the reference position. This kind or arrangement is commonly employed in door locks for returning the shaft of the door handle to its centered position. This invention was made to address a particular problem which occurs in operation of return spring arrangements in door locks and will be described with reference thereto as the illustrative embodiment of the invention. The problem in door locks which is solved by this invention may be found in other mechanisms where it is required to return the shaft to its reference position after the turning torque is removed.
The terms "center position" and "reference position" are used synonymously to mean that position of a shaft from which it is rotated and to which it is returned in a cycle of operation, whether it is rotated in only one direction or in either direction from such position.
In door lock mechanisms, the operating shaft carries a cam which actuates a slide and lever for retracting the bolt when the operating shaft is turned in either direction. A return spring is typically used to bias the slide or lever toward the position in which the bolt is extended. The return spring may be somewhat prestressed with the shaft in its center position and the return force which acts to bias the shaft toward its reference position is at its minimum value with the shaft in the center position and is at its maximum value when the shaft is turned to its limiting position in either direction. Because the return bias force diminishes as the reference position is approached, the shaft may not be fully returned to its center position because of friction or gravity loading in opposition to the return bias force. This condition is aggravated after many cycles of operation which results in some loss of resiliency in the return spring. Although the lock operation may not be significantly impaired, there is a possibility in some lock mechanisms that the return bias force may not be effective to return the mechanism to a position which allows the bolt to be fully extended. Further, in locks with a lever handle, the failure of the return spring to return the shaft to the center position is undesirably noticeable in that the lever handle droops from its normal horizonal position.
In electronically controlled locks, the failure of the return spring to return the shaft to the center position can be much more serious, even to the extent of loss of security. In certain electronically controlled locks, the operating shaft is prevented from turning by a solenoid actuated lock pin which enters a lock pin receiver mounted on the shaft when the shaft is in its center position and the solenoid is deenergized. In this kind of system, the door is locked with the lock pin extending into the lock pin receiver. It is unlocked in response to a key bearing the correct code which causes the electronic control system to energize the solenoid and retract the lock pin from the lock pin receiver. After the door is unlocked and opened by turning the handle, the solenoid is deenergized, allowing the lock pin to drop against the receiver. If the operating shaft is returned to the center position, the lock pin will enter the receiver and the door is relocked. If the operating shaft does not return to the center position upon release of the handle, the lock pin will be misaligned with the receiver and the lock pin will not enter the receiver and the door will remain unlocked.
In door locks and other such mechanisms, the problem of the return spring cannot be solved by simply using a stronger spring because there is a practical limit to the amount of turning force which should be required to operate the door lock. Since the return bias force provided by the return spring increases with angular displacement of the operating shaft, the maximum permissible torque places a limit on the strength or spring rate of the return spring to be used. Thus, the problem is that of providing the operating shaft with a return bias torque which is great enough to ensure that the shaft is returned to its center position and yet the operating torque for turning the shaft to its fully operative position must not exceed a predetermined maximum.
A patent search conducted prior to filing of this application did not reveal any prior art which addresses the aforesaid problem. No prior art is known to applicant which is pertinent to the invention set forth hereinbelow.
A general object of this invention is to provide a recentering means for an operating shaft which will ensure that the shaft is returned to its reference or center position without requiring a turning torque greater than a predetermined maximum value.